1. Field of the Invention
The present invention generally relates to an image forming apparatus for forming by use of an electrostatic latent image for example. To be more particular, the present invention relates to an image forming apparatus for forming monochromatic and color images by use of electrophotographic recording, electrostatic recording, ionography, or magnetic recording.
2. Description of Related Art
Recently, many color image forming apparatuses have been developed for commercial use. Especially, a color image forming apparatus having a plurality of image supporters for supporting toner image formation has been developed by making the most of its high image productivity along with a conventional color image forming apparatus for obtaining one piece of image by a plurality of rotations (for example, four rotations). Such a color image forming apparatus having a plurality of image supporters (this color image forming apparatus is hereafter referred to as a tandem-type color image forming apparatus) widely uses a belt-like toner image carrier for carrying the transfer of a toner image from the plurality of image supporters by circulating along these image supporters or a belt-like toner image carrier for carrying a toner support sheet for receiving the transfer of a toner image. These toner image carriers provides advantages that it is easy to form a transfer plane abutting a plurality of image supporters arranged generally linearly and an appropriate contact is obtained by use of belt flexibility without damaging the image carrier. At the same time, the toner image carriers of this type have a disadvantage that instability in belt carriage makes it very difficult to make alignment of color toner images, one of the most important factors for defining color image quality. These toner image carriers have another disadvantage that plural sets of color image forming units each composed of an image supporter and an image forming member around the image supporter result in an increased size of the entire image forming apparatus.
FIG. 17 shows an example of the constitution of a conventional image forming apparatus. Shown in the figure are four photosensitive drums 1, 2, 3, and 4 and a belt-like toner carrier 5 that is wound around a drive roll 6 and follower roll 7 to move along these four photosensitive drums 1, 2, 3, and 4. The photosensitive drums 1, 2, 3, and 4 are arranged thereabout with charging units 11, 21, 31, and 41 for uniformly charging the photosensitive drums, exposure units 12, 22, 32, and 42 for exposing the photosensitive drums to form an electrostatic latent image, developing units 13, 23, 33, and 43 for developing the electrostatic latent image with color toners to form toner images of these colors, and cleaning units 14, 24, 34, and 44 for removing residual toners from the photosensitive drums, respectively. The belt-like toner carrier 5 may be either a so-called intermediate transfer member that directly carries a toner image or a so-called transfer paper carrying member that sucks a toner image support sheet such as a transfer paper onto the belt-like toner carrier 5. Hereafter, the toner image carrier for carrying a toner image by directly supporting the same and a belt-like toner image carrier for carrying the toner image support sheet for supporting a toner image maybe collectively referred to as a transfer belt.
The following describes details of an image forming process. First, the photosensitive drum 1 is uniformly charged and then exposed. An electrostatic latent image formed by the exposure is developed with toner. A resultant visible toner image is transferred by a transfer unit not shown onto the transfer belt 5 at a transfer position contacting the transfer belt 5. When this first toner image comes to a position at which the first toner image contacts the photosensitive drum 2, a toner image formed likewise on the photosensitive drum 2 is superimposed as a second toner image onto the first toner image. Likewise, a third toner image and a fourth toner image are sequentially superimposed to form a toner image of four colors on the transfer belt. However, while the transfer belt travels along the four photosensitive drums, the travel speed of the transfer belt cyclically varies due to out-of-roundness or eccentricity of the drive roll 6 for driving the transfer belt. This causes a delicate shift in alignment of the toner images of the four colors, failing to provide a desired image. In order to prevent this problem from occurring, various methods of securing the alignment have been employed.
In one of these methods, an image defined for position sensing is developed and transferred, the position of this image is read by an image sensor, the position of each color is calculated, and the shift in alignment is corrected by correcting the exposure timing of the exposure units 12, 22, 32, and 42 or minutely adjusting the position of the reflection mirror of the light source in each exposure unit. However, this method requires a very complicated mechanism and therefore increases fabrication cost.
In order to avoid this problem, methods are disclosed in Japanese Published Examined Patent Application No. Hei 6-13373, Japanese Published Unexamined Patent Application No. Sho 62-205372, and Japanese Published Unexamined Patent Application No. Hei 1-31173 in which the distance between the transfer positions of the plurality of photosensitive drums 1, 2, 3, and 4 is set to a value equivalent to an integral multiple of a distance traveled by the transfer belt 5 when the drive roll 6 has made one full rotation. To be more specific, the relationship between distance L between transfer positions and diameter D of the drive roll shown in FIG. 17 is L=n .pi.D (n being an integer). According to this method, the phase angle of eccentricity of the drive roll 6 at the time of transferring the toner image of each color is made constant for all other toner images, thereby canceling the relative color offset in the image to be transferred.
Recently, the size of the apparatuses such as those mentioned above has been significantly decreasing. Therefore, application of the above-mentioned technique reduces the diameters of the photosensitive drums 1, 2, 3, and 4 as well as distance L therebetween as shown in FIG. 18. The diameter D=L/n .pi. (n being an integer) of the drive roll must also be made smaller. When the diameter of the drive roll 6 is made smaller, a deflection is caused in the drive roll 6 due to the tension of the transfer belt 5 wound thereabout, a problem not negligible. The deflection is inversely proportional to the secondary moment of area of the drive roll 6 itself, namely diameter D raised to the fourth power. This is graphically represented in FIG. 19. In FIG. 19, a dashed-line curve represents a maximum deflection with roll diameter D=10 mm being 100. A solid-line curve represents the secondary moment of area with roll diameter D=10 mm being 1. Thus, the graph of FIG. 19 indicates that the deflection varies drastically with the roll diameter. Therefore, as the diameter D of the drive roll 6 decreases as shown in the example of FIG. 18, the deflection caused in the drive roll 6 increases drastically. As a result, there occur problems in which waving caused on the transfer belt 5 disables uniform transfer or correct color alignment or which cause a wrinkle on the transfer belt to remain unremoved, thereby making the transfer belt unavailable thereafter. The waving on the transfer belt due to the deflection of the drive roll causes problems not only in color image formation but also in monochromatic image formation, in which poor transfer or, in the case of a transfer belt on which transfer paper or the like is put, failure of carrying the transfer paper is caused.
The above-mentioned Japanese Published Examined Patent Application No. Hei 6-13373 discloses an example in which a follower roll 7 around which the transfer belt 5 is wound and the drive roll 6 having a diameter matching the distance between transfer positions (the distance between photosensitive drums) are arranged separately as shown in FIG. 20. However, in this arrangement, the speed and position of the transfer belt 5 that actually passes the photosensitive drums 1, 2, 3, and 4 are affected by the eccentricity and out-of-roundness of the follower roll 7, so that, as with the drive roll, relationship of d=L/m .pi. (m being an integer) is also required for the follower roll 7 relative to its diameter d. Eventually, aligning the phases of the toner images of the colors requires the follower roll 7 to have diameter d=L/.pi. (m=1) at most. Therefore, if L is small, d must be small accordingly, thereby causing the problem of deflection due to the tension of the transfer belt 5.
If a load is applied to the transfer belt 5 by passing the same between the drive roll 6 and a pinch roll 8 as shown in FIG. 20, the load is actually applied to the shaft of each of the rolls at both ends thereof as shown in FIG. 21, thereby causing a large deflection on the drive roll 6 also of a small diameter. This causes a gap between the rolls at the center thereof, which in turn causes poor application of the load, resulting in poor transmission of carrying force or uneven pressure distribution, which may cause such troubles as wrinkles on the transfer belt. FIG. 22 shows a method in which a pressing member 57 for urging the drive roll 6 especially at the center thereof is arranged to decrease the deflection. However, in this arrangement, because a compressing force is applied to a part of the surface of the drive roll, that part is easily exposed to such troubles as scratch, dent, and wear, thereby significantly decreasing the durability of the drive roll 6. Another well-known method is that a uniform pressure is generated by the drive roll and the opposing member such as a pinch roll are urged to each other by mutually tilting these rolls by a small angle from parallel direction. However, this arrangement applies undue shear force to the belt, thereby causing damages such as wrinkle and crack on the belt and reducing the durability of the belt.